1. Field of the Invention
This invention relates to antennas, and more particularly to a method for calibrating a phased array antenna and a calibrated phased array antenna.
2. Description of Related Art
Microwave terrestrial and satellite communications systems are rapidly being deployed to serve communications needs. In these systems, to ensure a radio communication link between a fixed station on the ground or on a satellite and a mobile station such as an automobile or airplane, antenna systems with scanning beams have been put into practical use. A scanning beam antenna is one that can change its beam direction, usually for the purpose of maintaining a radio link, e.g. to a tower or satellite, as a mobile terminal is moving and changing direction. Another application of a scanning beam antenna is in a point-to-multipoint terrestrial link where the beams of a hub antenna or remote antenna must be pointed in different directions on a dynamic basis.
Early scanning beam antennas were mechanically controlled. The mechanical control of scanning beam antennas have a number of disadvantages including a limited beam scanning speed as well as a limited lifetime, reliability and maintainability of the mechanical components such as motors and gears.
Electronically controlled scanning beam antennas are becoming more important with the need for higher speed data, voice and video communications through geosynchronous earth orbit (GEO), medium earth orbit (MEO) and low earth orbit (LEO) satellite communication systems and point-to-point and point-to-multipoint microwave terrestrial communication systems. Additionally, new applications such as automobile radar for collision avoidance can make use of antennas with electronically controlled beam directions.
Phased array antennas are well known to provide such electronically scanned beams and could be an attractive alternative to mechanically tracking antennas because they have the features of high beam scanning (tracking) speed and low physical profile. Furthermore, phased array antennas can provide multiple beams so that multiple signals of interest can be tracked simultaneously, with no antenna movement.
In typical embodiments, phased array antennas incorporate electronic phase shifters that provide a differential delay or a phase shift to adjacent radiating elements to tilt the radiated phase front and thereby produce farfield beams in different directions depending on the differential phase shifts applied to the individual elements or, in some cases, groups of elements (sub-arrays). Of course, there is a need to efficiently and effectively calibrate the phased array antennas and, in particular, there is a need to efficiently and effectively calibrate phased array antennas that incorporate voltage tunable dielectric phase shifters. These needs and other needs are satisfied by the method for calibrating a phased array antenna and a calibrated phased array antenna of the present invention.
The present invention includes a method for calibrating a phased array antenna and a calibrated phased array antenna. In the preferred embodiment of the present invention, the method for calibrating a phased array antenna containing voltage tunable dielectric phase shifters and a controller for supplying control voltage to the phase shifters includes the steps of: (a) applying zero voltage to each of the phase shifters and measuring the phase of each of a plurality of columns of radiating elements in the phased array antenna; (b) using the measured phase to determine a phase target value for each of the plurality of columns of radiating elements in the phased array antenna; (c) adjusting a phase shift for each column of the radiating elements in the phased array antenna to a value within a predetermined range of the phase target value to generate phase offset data; and (d) using the phase offset data in a calibration table used by the controller to adjust the tuning voltage of each of the phase shifters to cause the columns of radiating elements to yield a uniform beam.